jeudi 3 février 2005

Nikkor AF 28-70mm Zoom... Plastic or Fantastic? In 1991 Nikon announced a £220 standard zoom lens which, while seemingly aimed at the amateur side of the SLR market by its price tag, drew more than the usual discussion about a "new Nikkor" from people in the know. The reason being was that for the first time Nikon had used a "plastic" element in a 35mm SLR lens! People who used the Nikon system for its high quality suspected, at first, that economy had gone too far. However, only a short time after the lens had been released was it realised that this was probably to be the way forward with new developments from Nikon's optical designers and engineers. But first a little relevant history... in 1968, Nikon used an "aspheric" element in a truly exotic optic designed for photographing skyscapes - the Fisheye-Nikkor 10mm f/5.6 OP. This lens was different from previous fisheye lenses because it produced a so-called orthographic projection (hence the "OP" designation) in which all parts of the circular image were of equal exposure - unlike regular fisheye lenses where exposure falls-off to the edges and corners. This was made possible by using a very costly hand-ground and polished aspheric front element. The exercise was extended in 1976 with the unique 58mm f/1.2 Noct Nikkor, an ultra-fast standard lens designed for available light photography at maximum aperture. Here the aspheric element eliminated coma so that bright point-sources of light would be reproduced accurately and without any image degrading flare or smearing in the near-dark conditions where it would be most used. Aspheric elements require high precision grinding and polishing because their complex curved surfaces do not conform to the regular shape of a sphere which is much simpler to cast, grind and polish in manufacture. The advantages of using an aspheric element are improved performance through the correction of wide-angle distortion and a reduction in the number of elements resulting in a smaller and lighter lens. This costly technology has been increasingly used in the past few years as Nikon has added several professional-grade autofocus, wide aperture, wide-angle prime and wide-angle zoom lenses to its already extensive catalogue. But then in 1991 Nikon broke the mould in an unexpected way with a mass-produced lens which was priced for the amateur photographer's pocket. The compact Nikkor AF Zoom 28-70mm f/3.5-4.5 zoom lens was marketed primarily to be sold with the previously announced F601 (N6006 in North America) SLR... the lens being small enough so as not to obstruct illumination coverage from the camera's pop-up flash at the wide-angle setting. Nikon was able to accomplish cost-effectiveness with this mass-market lens by using a new hybrid element. This was manufactured by applying a special ultraviolet cured resin onto a previously formed near-accurate aspheric glass surface which was then bonded together in a precision-ground metal casting. Although the resin used is, in fact, a coating of only a few microns, it is possible to mould it so that the edge thickness is slightly, but measurably, different from that in the centre of the newly formed element... so forming a new aspheric curved shape. These hybrid elements can be multicoated conventionally, just like their glass counterparts, so helping to improve light transmission and reduce flare while at the same time improving optical performance by reducing distortion to minimum, or certainly acceptable, levels. The use of a hybrid aspheric element also enables the lens designer to reduce the number of elements, thus reducing lens size and weight with a potential advantage of lower overall cost. The important question though is how does such a lens perform in practice? I must say, first of all, that I am not a natural user of zooms... I have always preferred to carry a couple of fixed focal length lenses... and having earned my crusts largely with Nikon equipment for over 30 years I've come to rely on faster, fixed aperture lenses of proven ability. Thirty years ago the few zooms available were of questionable quality, but not so today. Their only drawback now is that in the interests of compactness many zooms' maximum apertures have become slower and variable depending on the focal length range. While the 28-70mm fits into this category in having a variable maximum aperture of f/3.5 at 28mm and f/4.5 at 70mm the topic of "questionable quality" does not apply because of the aspheric element in its construction. And the old argument about not being able to accurately set an exposure manually doesn't apply here either... the lens is for everyday photography, not for the Zone System exponent! Having said that the Nikon system of exposure is second to none in terms of precision... its 3-D Distance Data Detection metering being the key. All recent autofocus lenses (plus some specialist manual ones) hold a microchip which relays the all-important subject-to-camera distance and combines it with a set of stored algorithms to provide the optimum exposure under many different lighting conditions. Therefore with with this lens on a compatible autofocus camera the user basically cannot get the exposure wrong . The autofocus speed is perfectly adequate for most uses this lens will be put to... noticeably faster autofocus is now available with Nikon's professional Silent Wave telephotos and wide-angle zooms, but for most photographers there is nothing to be gained, or even seen through the viewfinder, with the few tenths or hundredths of a second difference focusing between them. In manual focusing, which some prefer for critical use, the lens performs smoothly with a reasonably well-damped action made nicer by the arc of travel of only 120 degrees from the infinity setting to the minimum 0.35 metre distance. The minimum focus distance is marked as 0.5 metre but there is a "macro" mode clearly marked in orange at all focal lengths down to 0.39 metre. In case you wonder why an arc of travel of 120 degrees is a plus point I find that it allows a complete infinity-to-macro focusing movement to be completed in one movement of the thumb or forefinger by rotating the focusing ring without having to readjust one's grip on the camera and lens. I've never tested a lens against a paper test chart (my eyes are now too old for such critical interpretation) but can attest that the results from this 28-70mm Nikkor are of a quality most would expect from a set of prime lenses. OK, you are going to have to be working stopped down to around f/8 for the best resolution, whereas with an f/1.4 prime lens the optimum resolution would be reached by f/4 or f/5.6 thus enabling a faster shutter speed relative to the film speed in use. But having studied the results from lens tests in magazines, from the UK's "Amateur Photographer" to Italy's "Tutti Fotografi" all reviewers and testers say the same... the results from this lens are from "very good" to "excellent." There is some vignetting of around 1/3rd of a stop in the corners (against light toned areas) until the lens is stopped down to f/5.6 but throughout the length range the best apertures to use is f/8. Depending on testers' preferences for graphs or numbers the resolution indicators at this aperture are around 120 lpm (centre) and 90 lpm (edge) or 8.5 (centre) and 7.5 (edge) out of 10... very high figures indeed for such a reasonably priced lens. Manually stopping down modern lenses is becoming something of an old-fashioned practice nowadays with more cameras able to set apertures via control wheels once the lens is locked at its minimum aperture setting. I'm not suggesting that this increasing use of "Program" mode has resulted in the old aperture ring being neglected but they are becoming stiffer to operate, with jerkier clicks at the full-stop detents. At the front end, too, there is cause for complaint because of the old problem of a rotating front element cell... every time the lens is refocused a polarising filter would have to be realigned correctly with the sun to gain the maximum desired effect. In AF mode this is not a problem, but in manual focus mode two hands are needed - one to hold the lens in focus, the other to rotate the filter! Other than that the 28-70mm Nikkor is both excellent to use and easy on the pocket. For a dedicated "fixed" lens user such as myself it would not be such a difficult choice to buy one - which from me is a high recommendation indeed - but it is genuine because of the range of views it provides at the twist of the wrist. Focal lengths are marked at 28mm, 35mm, 50mm and 70mm on the zooming ring with markings for feet and metres indicated on the lens barrel... clearly an indication that Nikon is catering for the thinking as well as the casual photographer. I'm in both camps and would be happy with this 28-70 if I were to be restricted to owning only one lens for some reason... but it would be a difficult choice if I had more money to spend because the Leica 28-35-50mm Tri-Elmar would win any such contest! Although this budget-priced Nikkor has been out of production for a few years they are readily found on the second-hand market. I picked up a mint example for £100 whilst they were still listed in the Nikon catalogue... they may be cheaper nowadays. If so, try one and be surprised... I think this piece of plastic is quite fantastic. The few images I've chosen to illustrate what this lens does were taken on the shore of Aberdovey harbour in west Wales... this is just the type of subject and situation for a cheap zoom lens... when you have to watch your footing at the turn of the tide in such a location why risk an expensive lens with sand, silt and sea-water so close? BTW: If you want the most up-to-date Nikon news, reviews, tests, downloads, folios and opinions Nikon Links will have them listed and checked out. 7:40:39 PM    comment []